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Auto Correlation Analysis
Auto correlate temperature
- Apparent short term periodic behavior
- 200 year DomeC, 300 year Vostok
- Seems to be aliasing of seasonal variability
Apply smoothing
- Unambiguous 22K, 41K period (Vostok and DomeC)
- Modulated peaks are evidence for other periodicity
- Related to variability in Earth's orbit and axis
- Related to sums and harmonics of this variability
Common to temperature, CO2 and CH4
Notes:
The reason that seasonal variability is suspected as the cause of the short term periodicity seen in the record, is that the recent satellite record shows nearly a 5C difference between monthly global average minimum and maximum temperatures over a year. The ice core samples vary from 10-30 years apart for the first 20K years of the record. If a 15-year sample covers 14 winters and 15 summers, it will be about 0.3C too high. If it covers 14 summers and 15 winters, it will be -0.3C low. This is close to the variability seen in the data over the last 15K years and also explains why the period depends so much on the sample spacing and smoothing interval. The +/-0.3˚C uncertainty is based on the current hemispheric asymmetry, which at it's peak would result in well over +/-0.5˚C of uncertainty. Seasonally variable precipitation, which locks in the gases used to determine temperature, is also a factor. There may also be effects from short term sunspot variability. Lastly, the width of the blade slicing the ice core can remove an entire winter or summer season from the record.